Many sensors devices are available for the flow measurement of gases or liquids, depending on the desired application. Recently, due to rapid developments in the field of micro-electro-mechanical systems (MEMS) technology, numerous types of traditionally scaled sensor devices have been reduced in size, leading to the development of completely new market segments and applications. This trend of sensor devices miniaturization, coupled with opportunities in cost reduction, has led to demand for complete integrated sensor device solutions, which include signal conditioning, for example, by means of integrated circuits. As such, it has become highly desirable to try to develop two or even one chip solutions, which include the sensor components, sensor excitation, sensor signal processing and supply of an analog or digital output signal.
There is a particular need to have an integrated sensor solution in the field of flow sensing based on the principle of the thermal mass flow rate. Such sensor devices could include a heating element or resistor (heater) and a Wheatstone Bridge from thermistors (heating element sensor) which are arranged in such a way that a flowing medium transports heat energy from the heating element (heater) to the heating element sensor. This produces a voltage difference, which, after signal conditioning, represents a measure for the liquid or gas flow rate. However, in order to make signal conditioning possible, the difference between the temperature of the heater and the ambient temperature must be well-known or even better constant.
Therefore, in order to know the ambient temperature, a further temperature sensor (ambient temperature sensor) is necessary to determine the ambient temperature. Current solutions use two resistors with the same temperature coefficient (TC), one for the heating element, and one for the ambient temperature sensor. The resistance value of the heating element is determined on the basis of the available operating voltage and the necessary thermal output. The resistance value of the ambient temperature sensor is chosen to be a multiple of the resistance value of the heating element, in order to avoid self heating. Both resistors are part of a control loop where they are supplied with regulated currents, which are constant in ratio. Through the variation of the absolute size of these currents the control loop adjusts itself to an operating point, which guarantees a constant temperature difference between the heating element and environment, independently of variables like supply voltage, thermal resistance of the MEMS sensor, the type of flowing medium, or the ambient temperature. While the heating element, the heating element sensor, and the ambient temperature sensor are integrated into today's MEMS sensor devices, three further external (non-integrated) resistors are necessary in addition to an operation amplifier for controlling the loop. These further resistors are discretely implemented, since they must be adjustable, in order to compensate the tolerances of the MEMS sensor devices.
Unfortunately, the discrete resistors and their complex adjustment as part of the finished sensor module is holding back a cost efficient two or one chip solution. Therefore, it would be desirable to have an integratable, low-cost circuit to adjust and calibrate a MEMS sensor device for flow measurement of gases or liquids, thereby enabling an integrated MEMS sensor solution.